Почему Singleton прячет зависимости?

public class SingletonService {
    private static SingletonService singletonService;
    private FirstDependency firstDependency;
    private SecondDependency secondDependency;
 
    private SingletonService() {
    }
 
    public static SingletonService getInstance() {
        if (singletonService == null) {
            singletonService = new SingletonService();
        }
 
        return singletonService;
    }
 
    public void doSomething() {
        firstDependency.doSomething();
        secondDependency.doSomething();
    }
 
    public void setFirstDependency(FirstDependency firstDependency) {
        this.firstDependency = firstDependency;
    }
 
    public void setSecondDependency(SecondDependency secondDependency) {
        this.secondDependency = secondDependency;
    }
}
 
class Demo {
    public static void main(String[] args) {
        SingletonService singletonService = SingletonService.getInstance();
        singletonService.setFirstDependency(new FirstDependency());
        singletonService.setSecondDependency(new SecondDependency());
 
        singletonService.doSomething();
    }
}